I'm not exactly an expert on Start.

Not sure about the entrance. I just went to the main one.

You will get much better xp when the mobs become red to you instead of skulls. I'm experiencing this in SM cathedral at the moment. At 20 my XP started at about 266 (If i remember rightly). At 31 I am now seeing about 377 xp per kill.

If you have the scarlet key from scarlet monastery library you can unlock the doors to startholme.

You can also get a key drop the key to city that allows you to unlock gates between live side and undead side (I believe).